I am looking to preform a release study on iron oxide particles from a drug carrier, we have always used dialysis devices for other types of drug studies but I am slightly worried about the size of my nanoparticles and finding the right MWCO for a device. My nanoparticles are around 6.8nm in diameter. Is there a way to approximate this as a MW or is there a generally accepted pore size that correlates to diameter?
Here's a chart that compares protein molecular weights to pore sizes of dialysis and ultrafiltration membrane.
https://www.repligen.com/products/dialysis/pore-size-chart
Judging from the chart, for 6.8 nm particles, you will want to use a molecular weight cutoff of about 5,000 Da to be confident of retaining the particles, or at least 300,000 to be confident of them passing through. Molecular weight cutoffs are not sharp or exact, so it is important to allow plenty of leeway.
5,000 Da of MWCO will slow the dialysis process, you can use up to 25 kDa. I assume the 6.8 nm is a "dry" size, or is it hydrodynamic size?
Dorian Foster The calculator in the Malvern Zetasizer gives a molecular weight of about 59 kDa for a globular particle 3.4 nm in radius (= 6.8 nm diameter). I suspect you're talking about your iron oxide particles though and these have a molecular weight depending on the type (FeO, Fe2O3, Fe3O4). This isn't analogous to the MW (or molecular mass) of a protein. If you have Fe3O4 (magnetite), then the magnetic properties will enable you to effect a separation.
First, don't do dialysis. It takes forever and is messy (but fairly cheap). Use spin filters for small scale work and tangential flow filters for preparative work. They come in a large number of volumes and nominal pore sizes. We use this type a lot: Amicon® Ultra Centrifugal Filter, 10 kDa MWCO
As for the pore size: You need to start with an estimate as the previous answers indicated, then you have to try some different cut-off values. The nominal cut-off values of the filters are calibrated to proteins AND/OR linear polymers. Polymers and proteins scale very differently for their Mw-Diameter values so don't put too much faith in the estimates. Spin filters are in my experience sharper than dialysis filters but still fairly poor in size discrimination so what you should use depends a lot on what you want to separate and how complete the separation has to be.
An example: BSA (Mw 67 kDa) has a hydrodynamic diameter of just below 7 nm and it passes around 50% through a 50 kDa filter.
If the filters are not good enough you may have to go to size-exclusion chromatography. Slow and expensive but very precise.
If you can give some more info of what you want to retain I might give some more advice.
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